When allocating funds to combat invasive plants that threaten both biodiversity and agriculture, one important question needs to be answered. How can we determine whether a non-native species will become a problem if left unchecked?

In cooperation with the state Department of Agriculture and Montana State University, UM plant biologist Anna Sala has attempted to answer this question using saltcedar — a type of tamarisk — that has been spreading along riparian corridors in eastern Montana since the 1960s.

Saltcedar is a tough, woody shrub well-adapted to salt marshes, estuaries and dry, extreme climates. A native of Europe and Asia, it has widely invaded the green zones along waterways in the southwestern United States, displacing virtually all native plants in some areas and creating two-mile-wide thickets along scarce water resources.

“You need a bicycle helmet to crawl through some of those thickets,” Sala says.

As a postdoctoral fellow at the University of Nevada-Las Vegas, Sala demonstrated that while saltcedar has no greater intrinsic capacity to use water than native plants, it grows so much more densely in non-native habitats that it can dry up wet areas to the detriment of native species.

Knowing that saltcedar has radically changed riparian areas in the Southwest, land managers in Montana would like to know if saltcedar is a threat in Montana’s cold climate.

“The fact that we find saltcedar growing in Montana made us wonder whether the plant is resistant to colder weather,” Sala says. “If so, this would mean that the plant might have the potential to colonize this northern habitat.”

To learn about saltcedar’s invasive pattern and how it affects other plants in a community, graduate student Jason Sexton sampled riparian corridors in eastern Montana during the 1998 growing season. He found that the oldest saltcedar colonies are in southcentral Montana, and that wherever saltcedar is found, there are fewer native species, such as willow or cottonwood.

Furthermore, results showed that saltcedar apparently takes advantage of disturbed areas, such as overgrazed land, to displace native vegetation.

Sala and Sexton also conducted greenhouse experiments with saltcedar seeds and seedlings to determine whether genetic differences were accounting for its success in a colder climate.

They planted seeds from the Southwest and eastern Montana in two rooms that mimicked the average temperatures of each locale. The seedlings grew the same in the warmer temperatures, but only the seeds from Montana produced hardy plants in the colder temperature. The reason appears to be a greater investment in roots than in foliage, which would be killed off in the winter anyway and is a genetically controlled trait.

“We think the results are significant because they show Montana seedlings can cope with a colder environment and are genetically different from the Southwestern variety,” Sala says. “There seems to be enough variability in the plant’s genetic makeup to enable it to find the right genotype to establish in a new climate.”

Sala agrees that her work is not definitive but cautions that even though saltcedar is not the pest that Russian olive has become, it does bear watching. “It’s very difficult to define the invasive potential of a non-native species,” she says.

“Saltcedar is unique because we know it has become a huge problem in other parts of the country, but it’s still not dominant here. What will happen is worth investigating. It could be that it’s still in a lag phase, getting ready to erupt.”

— Caroline Lupfer Kurtz